improve json encoding error
Added {GET,DELETE} /admin/files

Previously, HandleWcFile() (nee 'wc_file()') was in charge of creating
its own Cache. Now main() creates the cache, which allows the new
HandleAdminFiles() handler to share the same cache.

GET    http://service/admin/files -- get the full cache contents
DELETE http://service/admin/files -- dump the cache

Drive-by:
  * renamed wc_file -> HandleWcFile
  * renamed Cache -> WcCache
  * acceptance test doesn't need to plumb $port around so much
  * acceptance tests can hit a different endpoint other than /files
"test.sh --quiet" is quiet